JP3753550B2 - Smoothing processing device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a smoothing processing apparatus that smoothes an image in, for example, a digital copying machine and a facsimile.
[0002]
[Prior art]
A conventional digital copying machine is configured to calculate an edge amount based on local information of a document in which characters and pictures are mixed, and to perform an appropriate filtering process on an image based on the edge amount. . For example, Japanese Patent Publication No. 6-5879 uses an edge detection filter that does not respond to halftone dots but responds to character edges, and mixes the output of the smoothing filter and the edge enhancement filter based on the detection signal. Thus, there has been proposed a method for preventing the occurrence of a defect called moire in the halftone dot portion by applying a smoothing filter process to the halftone dot portion.
[0003]
Japanese Patent Application Laid-Open No. 8-181864 discloses a problem called moire in a halftone dot portion by using a halftone dot degree calculation filter that responds to a halftone dot and smoothing it based on the detection signal. A method for preventing this has been proposed.
[0004]
[Problems to be solved by the invention]
By the way, when a relatively strong smoothing process is performed on the low density portion of the halftone dot as shown in FIG. 7A, the density data level of the halftone dot portion becomes as shown in FIG. There is a problem in that the image becomes substantially uniform at a low portion, and an unstable image depending on the printer, specifically, “a mottled low-frequency noise or a density decrease associated therewith” occurs. In other words, the instability of the image can be eliminated if the halftone dots remain on and off as shown in FIG. 7B. Such a destabilization phenomenon is more likely to occur in electrophotographic printers.
[0005]
The present invention has been made in view of such problems of the prior art , and its purpose is in a low density portion where dots are crushed when normal smoothing processing is performed in a halftone dot region. The purpose is to make it possible to generate a stable image without losing dots .
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the first means calculates a pixel density average value from the density of the target pixel of the input image and the density of pixels in the vicinity of the target pixel, and the pixel density average value is a predetermined value . If it is less than the threshold TH, the feature amount α is calculated as α = (255 / TH) × pixel density average value. If the pixel density average value is equal to or greater than the predetermined threshold TH, the feature amount α is calculated as α = 255. Feature amount calculation means, halftone dot area determination means for determining whether each pixel of the input image is a halftone dot area, smoothing by performing a smoothing process on the input image and the input image using a smoothing filter A smoothing processing apparatus comprising: a mixing unit configured to receive an image and generate an output image by mixing the input image and the smoothed image, wherein the mixing unit includes a halftone dot by the halftone dot region determination unit. Halftone dot determined to be a region For the region, by the feature amount α ,
Output image = {(255−α) × input image + α × smoothed image}
As described above, an output image is generated.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. 1 is a block diagram showing an embodiment of a smoothing processing apparatus according to the present invention, FIG. 2 is an explanatory diagram showing filter coefficients and processing of the smoothing filter circuit of FIG. 1, and FIG. 3 is a feature amount calculation circuit of FIG. It is explanatory drawing which shows the process of.
[0011]
In FIG. 1, an image signal input unit 1 is a scanner having a CCD element or the like, and reads a document image to generate an 8-bit per pixel substantially linear reflectance image signal m and a smoothing filter circuit 2 and a log conversion circuit. 3 and the mixing circuit 5. The smoothing filter circuit 2 smoothes the image signal m input from the image signal input unit 1 with a filter coefficient of a matrix of 7 × 5 pixels as shown in FIG. 2, for example, as shown in FIG. Performs relatively strong processing, and outputs the processed signal n to the mixing circuit 5. This filter coefficient is experimentally selected so that the halftone dot is substantially flat for a general halftone image (150 to 250 lines) in consideration of the scanner characteristics of the image signal input unit 1. That's fine. By this processing, it is possible to suppress the occurrence of moiré in the subsequent output device.
[0012]
In the present invention, the density level of the pixel of interest is calculated as a feature amount for controlling the smoothing degree. As preprocessing for this, the Log conversion circuit 3 converts the reflectance linear image signal m input from the image signal input unit 1 into a density signal (= 8 bits = 0 to 255). Then, the subsequent feature amount calculation circuit 4 obtains an average value of density values of neighboring pixels including the pixel of interest at the center in a 5 × 5 pixel matrix, for example, based on this density signal, and this average value is shown in FIG. With a one-dimensional table, in the low concentration area (0 to TH),
α = (255 / TH) × average value medium to high concentration region (TH to 255)
α = 255
And output to the mixing circuit 5 as a feature amount α.
[0013]
The conversion table shown in FIG. 3 has a conversion characteristic such that the smoothing degree for the low density portion of the image is weakened in the mixing circuit 5.
Output signal = {(255−α) × m + α × n} / 255
, The image signal m input from the image signal input unit 1 and the signal n processed by the smoothing filter circuit 2 are proportionally distributed based on the feature quantity α, and the processed signal is output to an image such as a printer (not shown). Output to the device.
[0014]
As another feature amount for controlling the smoothing process, it is effective to limit the above-described feature amount calculation target to a halftone dot region. In other words, it is possible to prevent the smoothing process from being performed on medium and high density areas, such as photographic paper and characters, where there is no possibility of moiré.
[0015]
FIG. 4 shows, as a modification of the feature amount calculation circuit 4, a configuration for calculating feature amounts so that the response is high in the halftone region and the non-low density region. Similarly, the density detection circuit 4-1 obtains an average value of density values of neighboring pixels including a central target pixel in a 5 × 5 pixel matrix, for example, based on the density signal from the Log conversion circuit 3. Then, the halftone dot area detection circuit 4-2 is based on the density signal from the log conversion circuit 3 by a known method as shown in, for example, Japanese Patent Laid-Open Nos. 2-112077 and 2-113378. Detect halftone area. The And circuit 4-3 outputs the detection signal of the density detection circuit 4-1 in the case of the halftone area, and outputs “0” in the case of the non-halftone area. Similarly, the table conversion circuit 4-4 converts the output value by a one-dimensional table as shown in FIG. 3 and outputs it to the mixing circuit 5 as a feature quantity α.
[0016]
Next, a second embodiment will be described with reference to FIGS. Here, as a usage form of the copying machine, there are many cases called a so-called generation image (grandchild copy) in which output paper is output again as a document. In general, a printer of a copying machine often performs halftone processing before output. If the halftone processed document also has periodicity, it is a document that may cause sampling moire. is there.
[0017]
Various methods have been proposed for dithering, but it was intended to reproduce dots with a large and large period in the low density area and with smaller dots and a small period in the medium to high density area. A scheme has been proposed. When such a reproduced image is output again as an original, it has been empirically known that the dots in the low density portion can be reproduced stably with less moire when they are moderately blurred (broken). On the other hand, it is important that the middle to high density portion does not generate moire by performing a stronger smoothing process.
[0018]
Therefore, in the second embodiment, the image signal input unit 1, the smoothing filter circuit 2, the log conversion circuit 3, and the feature amount calculation circuit 4 have the same configuration as that of FIG. 1, but the second smoothing filter circuit 6 And a selection circuit 7 are provided. As shown in FIG. 7B, the second smoothing filter circuit 6 performs processing with a lower degree of smoothing than the first smoothing filter circuit 2 and has a smaller matrix than the smoothing filter circuit 2. The image signal m input from the image signal input unit 1 for smoothing is smoothed by a 3 × 3 pixel matrix smoothing filter as shown in FIG.
[0019]
The selection circuit 7 selects the output of the second smoothing filter circuit 6 in the low density region (0 to TH) in which the feature amount α calculated by the feature amount calculation circuit 4 is less than or equal to the threshold value TH. In the high concentration region (TH to 255), the output of the first smoothing filter circuit 2 is selected and output. In addition, in order to simplify description, although the case where there were two smoothing filter circuits was demonstrated, in this case, when unnaturalness is conspicuous at the switching boundary of the smoothing degree, more types of smoothing filters This can be prevented by providing a circuit and switching according to the feature amount α.
[0020]
【The invention's effect】
As described above, according to the present invention, even in a halftone dot region, it is possible to reduce the degree of smoothing processing in a low density portion where dots are crushed when normal smoothing processing is performed. Thus, a stable image can be generated without losing dots .
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of a smoothing processing apparatus according to the present invention.
FIG. 2 is an explanatory diagram showing filter coefficients and processing of the smoothing filter circuit of FIG. 1;
FIG. 3 is an explanatory diagram showing processing of a feature amount calculation circuit in FIG. 1;
4 is a block diagram showing a modification of the feature amount calculation circuit of FIG. 1; FIG.
FIG. 5 is a block diagram illustrating a smoothing processing apparatus according to a second embodiment.
6 is an explanatory diagram showing filter coefficients and processing of the second smoothing filter circuit of FIG. 5. FIG.
FIG. 7 is an explanatory diagram showing a smoothing degree.
[Explanation of symbols]
2,6 Smoothing filter circuit 3 Log conversion circuit 4 Feature amount calculation circuit 5 Mixing circuit 7 Selection circuit

Claims (1)

A pixel density average value is calculated from the density of the target pixel of the input image and the density of pixels near the target pixel. When the pixel density average value is less than a predetermined threshold TH, α = (255 / TH) X a feature amount calculating means for calculating a feature amount α as a pixel density average value and calculating the feature amount α as α = 255 when the pixel density average value is equal to or greater than a predetermined threshold TH ;
Halftone dot region determining means for determining whether each pixel of the input image is a halftone dot region;
A mixing unit that receives the input image and a smoothed image obtained by performing a smoothing process using a smoothing filter on the input image, and generates an output image by mixing the input image and the smoothed image;
A smoothing processing apparatus comprising:
For the halftone dot area determined to be a halftone dot area by the halftone dot area determination means, the mixing means uses the feature amount α ,
Output image = {(255−α) × input image + α × smoothed image}
As a smoothing processing apparatus , an output image is generated.

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